Trouble Shooting Sunn SX8350 Powered Mixer

[Jesse Pearson]

Has anybody had good luck buying Service Manuals from "MusicParts.Com". Looks like they might have the right documentation for $20.00. Does this sound about right? Sunn Schematics

[Jazz P Bass]

Typically they have schematics.

You have the power amp schematic.

If you want the preamp, there you go.

[Jesse Pearson]

O.K. Enzo, you have me convinced to just get the amp working and stable first. That sounds like good advise. Thanks

[Enzo]

OK, I am now at my shop, but you folks have already come up with the power amp schematic, and that is all you need to fix the power amp really. I can try to scan the other parts of the schematic, but if they are 11x17, that is more than I want to do, chopping them into parts. But if we need just a portion...

If you call Fender, they would likely email you the whole schematic for the model.

Your amp ran hot from the start? The bias was probably misadjusted from the start.

Look in your schematic, near the middle. Looks like Q15, says 22-0067 next to it. That is your bias transistor, and near it are R40 and R42. R42 is the lower one and is a trim pot labeled TRIM. That is your bias control. The lower the resistance setting of R42, the colder the amp runs.

The trick Jazz refers to is to simply monitor the mains current draw at idle. Turn the control and watch the current. if it starts to increase, turn it the other way. Turn it down to the point the mains current no longer gets lower. That is as close to the proper bias as the amp needs to be.

If you look bottom center of the drawing, it says right on it "bias setting". I like my method better.

I just looked in my Sunn book, there is the one page power amp schematic, and the one page preamp schematic (the mixer itself). That's it. Amps like this do not get large detailed manuals.

Here is a bulletin on it. The bulletin includes other model issues too. Before launching into any of it, make sure the changes are not ALREADY made. They implemented the updates during production, so you may already have them.

[Jesse Pearson]

Way cool Enzo, I see that on the schematic. Q15 was one of the transistors that blew and I got a replacement from NTE at Frys. I was just studying on the internet about how and what the bias was and how to adjust it specifically with a class AB amp. Today was a good day, because I learned some good knowledge and made some new friends. Thanks guys

[Enzo]

When I work on a power amp like this, honestly, I don't even think about what class the thing is designed for. Kinda like when i am driving my car, I don't think about the compression in the cylinders or the bore and stroke.

[g1]

As far as the plastic covers that you thought were protecting the transistors, I think those were probably the "clamps" or retainers? They hold down the transistors tight to the heatsink, so perhaps removing them allowed the transistors to overheat.
The "gaskets" are insulators. They prevent the metal parts of the transistors from making electrical contact to the heatsink, which is usually grounded to chassis. The heatsink grease/paste, or thermal compound, assures a good thermal connection to the heatsink. The stiff, clear (or translucent) types are mica and use the grease. There are also a more pliable, rubbery type, called sil-pads that do not use the grease. They are usually grey.

[Jazz P Bass]

If this amp was running hot from the get go, loosening the contact to the heatsink was certainly suicide to the output transistors.

[Enzo]

The covers are to cover the transistors, like on the old PV 400 series.

There is a photo of the rear you can see here:
Used Sunn SX8350 Powered Mixer | Mixers | Music Go Round

The output transistors are mounted on heat sinks on the rear panel. The transistors would be exposed but for the plastic TO3 covers.

Crappy picture of cover:
Keystone Electronics - 4632 - Nylon TO-3 Transistor Case Cover - Allied Electronics

[Jesse Pearson]

Looked at the bias circuit schematic again with a magnifier glass and noticed what we thought was Q15 is really Q13 (bias circuit), the 3 looks like a 15 from the thick print. Q15 and Q6 (low power transistors) both blew their faces off but I found the pieces in the p.a. box and pieced them back together so I could get the parts numbers off of them and find the replacement part numbers for them for a parts house. So this brings me to this question (admittedly I could have been doing something wrong), the numbers next to the Sunn Power Amp schematic transistors don't seem to connect to any parts distributor that I could find and I was wondering if that is why there is a parts lists you can buy when buying a service manual? Does this mean if a part like a low power transistor self destructs to where you can't read what it said on the power transistor part wise and you don't have a parts list then your stuck in the mudd when trying to replace the part? If not, how should I have read Q15 and Q6 on the power amp schematic to cross reference a parts house for the right replacement part? Thanks

[Jesse Pearson]

I only removed the plastic covers to conduct the freeze spray test on the outside casings of the transistors, which the spray could have gone into the empty screw holes and caused a short. Or it could have been one of the power transistors started to come out of it's pin holders on one side of the transistor when I moved the p.a. box to the side of my work table and that caused a short. Don't know, but I'll never do that again.

[Enzo]

The numbers next to the parts are Sunn part numbers. So no supplier will have them by those numbers.

Look at your schematic, along the top. See the pictures of transistors with the leads identified? The small plastic ones have maybe 8 in a list. Note the numbers. Now look at the schematic numbers. Q6 is 22-5089, now look at the list. See 2N5089? Now Q15 is 22-4250. Check the list to find 2N4250. Those are what those parts are really.

Have you called Fender yet to ask if they will email the manual to you?

If I had no idea what those types were, I would look at comparable amps and see what was used. All these solid state power amps are similar under the skin.

2N5089 is a 25v 100ma NPN TO92 at Q6. Most anything similar will work. Certainly the 2N5088 would, it is the same but for a 30v rating. The 2N4250 will have similar specs but PNP. Peavey likes to use the MPS6533 in that application, for example. it is a 40v 600ma part, but that works fine.

[Jesse Pearson]

Great, thanks Enzo. Now I see what you mean. I will call Fender tomorrow.

[Jesse Pearson]

Well I found some errors with the silk screen component numbers and the schematic. The circuits on the board all line up with the schematic, but some of the silk screen numbers (transistors) on the board are not the right component and circuit locations as the schematic. For example, there are two Q14 locations with the silk screen. So I am having to follow the circuit traces with the schematic and mark corrections on the board. It's cool, bet this can happen a lot when companies use the same board for closely related modals. My modal SX8350 was purchased in 1985 I think. The schematic says "Rev B" "1985". Q3 on the schematic is silk screened Q12 and Q12 on the schematic is silk screened Q14. I'm having to line up all the transistor circuit components for the various Q locations on the schematic and find the same circuit and component structures with the surrounding resisters and cap numbers ( which seem correct silk screen wise) on the board along with the trace road map from the board. So it looks like just the Q transistor numbers on the board are messed up here and there My friend who used to teach electronics years ago pointed this out to me. After replacing some more caps and reinstalling a backwards low power transistor I'm still getting distortion and no audio power with the limit light on the p.a. panel coming on and staying on for almost a minute before going out. I get a big thump from the speakers when turning the power off. The investigation continues...

[Jesse Pearson]

Good news, got the P.A. working again. Set the Bias and there is almost no heat after hours of pumping music through it. Can't believe how much I learned working on this thing. My p.a. was manufactured before Fender came out with the thermal modification updates which came out two years later (thanks Enzo,). The power amp schematic posted above was printed after the mods were made. There were some silk screen printing issues on my circuit board that were different then the updated schematic posted above in a earlier post. This caused some confusion when troubleshooting and thankfully my ex-electronics teacher buddy showed me how to trace the boards trace's and components out and compare it to the schematic. Q3 on the updated schematic above is Q12 on older silk screened boards. Q12 on the updated schematic is silked screened Q14 on older boards. R17 on the updated schematic is R47 on older boards. R47 on the updated schematic is R17 on older boards. Since I already have new replacement electrolytic caps for the pre-amp section (mixer) my friend wants me to replace them because they are 30 years old. I'm also starting to do the recommended thermal mods that Fender suggested and I'm putting in a newer and longer 10 pin ribbon connectors and strip to make any future servicing much easier. **One big lesson I learned by working on this project is to not let anyone interrupt you when your installing different transistors etc. I reinstalled a couple of transistors in the wrong order that were right next to each other, and also put a three prong transistor in the wrong order. This caused some cascading to take place on fire up, no flames but new failed components. My electronics buddy really likes the Sunn p.a. with the way it sounds through the 15" speakers and would like to find one for himself. Thank you everyone for your help, especially Enzo.